This invention relates to a device for coupling radiofrequency (RF) power to a magnetically confined plasma. RF power is used to excite waves of the frequency corresponding to the lower hybrid resonance of a plasma. RF induced lower hybrid waves can be used to heat or drive current in the plasma. A phased waveguide, such as the one described in U.S. Pat. No. 4,110,595, can be used for launching RF power to heat and drive currents in toroidal plasmas.
The RF waveguide is inserted through the walls of the plasma containment device and is positioned such that the waveguide mouth is near the edge of the plasma (the edge of the plasma is that part of the plasma near the boundary or surface of the plasma). RF coupling depends critically on the electron density immediately in front of the waveguide mouth. In overdense plasmas (.omega.pe.sup.2 &gt;&gt;.omega..sup.2, where .omega..sub.pe is the electron plasma frequency and .omega. is the wave frequency) the reflective properties of waveguides operating in the lower hybrid regime of frequencies are essentially determined in the narrow layer near the edge of the plasma, in which .omega..sub.pe.sup.2 .perspectiveto..omega..sup.2. Density modifications in this critical region may correspondingly have a strong effect on the reflection.
Optimum coupling occurs when n.sub.e .perspectiveto.n.sub.c n.sub..parallel..sup.2, where n.sub.e is the plasma edge density, n.sub.c is the critical density, and n.sub..parallel. is the (parallel) refractive index. Varying the edge density from the optimum by an order of magnitude can increase the power reflection by a factor of 2 to 3 and can lead to waveguide arcing.
Therefore, it is an object of the present invention to improve RF coupling to a magnetically confined plasma.
It is another object of the present invention to modify the edge density of a magnetically confined plasma.
It is yet another object of the present invention to reduce the reflection coefficient of the waveguide and to reduce waveguide arcing when RF power is applied to a magnetically confined plasma.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention.